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The present invention relates generally to micro-mechanical switches and relays, and more specifically to electro-statically actuated micro-switches and micro-relays that have reduced susceptibility to malfunction and/or damage from electrostatic discharge. The present invention further relates to protection devices for protecting electro-statically actuated micro-switches and micro-relays from adverse effects of electrostatic discharge.
Micro-mechanical switches and relays (also known as micro-switches and micro-relays) are frequently employed to route signals in electronic devices and systems. Such micro-switches and micro-relays typically comprise at least one source terminal, at least one gate terminal, at least one drain terminal, and various style beams mounted to a substrate. A respective beam is controlled by the gate terminal for selectively connecting the source terminal to the drain terminal (as in a conventional micro-switch device) or for selectively connecting two (2) drain terminals (as in a conventional micro-relay device). Such micro-switch and micro-relay devices are described in U.S. Pat. No. 6,153,839 issued Nov. 28, 2000 entitled MICRO-MECHANICAL SWITCHING DEVICES.
In recent years, micro-switch and micro-relay devices have become increasingly susceptible to malfunction and/or damage from application of excessive voltages resulting from, e.g., electrostatic discharge (ESD). This is due at least in part to the decreasing sizes of the devices as circuit geometries are scaled down.
During the manufacture and assembly of micro-switch and micro-relay devices, all terminals of the devices are typically ESD-sensitive. For example, electrostatic charge may be transferred from one or more terminals of a micro-switch or micro-relay device to another conducting object during the manufacture or assembly of the device. This charge may generate voltages that are large enough to cause the device to malfunction, breakdown the gate oxide on the device, or dissipate sufficient energy to produce electro-thermal failures in the device.
Moreover, when micro-switch and micro-relay devices are being used in the field, one or more drain terminals of the devices are often used to connect internal systems, which may be implemented on the same substrate as the device, to external systems. This exposes the drain terminals to potential ESD that may cause malfunction in and/or damage to the devices.
One approach to reducing the adverse effects of ESD during the manufacture and assembly of micro-switches and micro-relays is to require special handling of these devices. For example, such special handling may involve proper grounding of manufacturing and assembly personnel who come in contact with the devices, and/or the use of ESD-resistant containers for transporting the devices during the manufacturing and assembly processes. However, such specialized device handling is often expensive to implement and may increase manufacturing and assembly costs.
Another approach to reducing the effects of ESD not only during the manufacture and assembly of micro-switches and micro-relays but also when these devices are in-use is to have ESD protection built into the integrated circuits containing the devices. However, conventional ESD protection techniques used in integrated circuits, in general, are not always suitable for protecting micro-switch and micro-relay device structures. This may be because of constraints in the manufacturing process, or because the conventional ESD protection techniques may disturb the signal to be switched.
It would therefore be desirable to have devices such as micro-switches and micro-relays that are less susceptible to the adverse effects of ESD. Such devices would have reduced susceptibility to malfunction and/or damage from ESD during the manufacture and assembly of the devices and when the devices are being used in the field. It would also be desirable to have ESD protection devices that are suitable for protecting micro-switches and micro-relays.
Apparatus and methods for protecting devices such as micro-switches and micro-relays from adverse effects of electrostatic discharge (ESD) are provided. In a first embodiment, a protection device is provided that includes a two (2) terminal switch that can be actuated by an ESD event to protect an ESD-sensitive micro-switch or micro-relay from potential malfunction and/or damage. The two (2) terminal switch is configured to close in less time than the micro-switch or micro-relay it is protecting, thereby dissipating the energy associated with the ESD event without causing damage to the micro-switch or micro-relay. In one embodiment, the two (2) terminal switch includes a source terminal, a drain terminal, and a beam mounted on a substrate. The drain terminal includes at least one drain contact, and the beam includes a first end attached to the source terminal and a second end overhanging the drain contact to define a drain contact overlap area. In a preferred embodiment, the drain contact overlap area of the protection device is configured to allow the two (2) terminal switch to close at a predetermined voltage and in less time than the micro-switch or micro-relay it is protecting.
The at least one drain contact of the protection device is configured to withstand the potentially damaging effects of the ESD event. In one embodiment, a respective current-limiting resistor in series with the at least one drain contact is employed in the protection device. In another embodiment, the protection device includes at least one first drain contact in a primary conducting path, at least one second drain contact in a high resistance path in parallel with the primary conducting path, and a respective current-limiting resistor in series with the at least one second drain contact. Respective drain contact overlap areas associated with the first and second drain contacts are configured to allow the high resistance path to close before the primary conducting path, thereby allowing the primary conducting path to close safely to provide a low resistance path for current flow. In alternative embodiments, increased numbers of drain contacts and/or different drain contact compositions are used in the protection device to resist damage from the ESD event.
In still another embodiment of the protection device, a gate terminal is employed to bias the two (2) terminal switch to make closure of the switch faster. The gate terminal can be further employed as a third terminal to maintain actuation of the two (2) terminal switch while the drain terminal removes excess electrostatic charge.
In a second embodiment, micro-switches and micro-relays are provided with increased immunity to the adverse effects of ESD events. The micro-switch includes respective drain/gate terminal pairs at respective ends of the device. The micro-relay includes at least two (2) drain terminals and a gate terminal at respective ends of the device. The micro-switch and micro-relay are configured to be less sensitive to ESD events by using the gate terminal at one end of the device to latch that end of the device down, thereby increasing the gate or drain voltage required to generate a threshold electric field to pull the other end of the device down. In alternative embodiments, this gate or drain voltage is further increased by adding contacts and/or bumpers to prevent contact between the beam and the gate terminal at the latched-down end of the device.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.